Generally speaking, there are two broad categories of ground-based air-pollution monitors: (i) point monitors and (ii) open-path monitors.
Most air-pollution measurement systems employ point monitors. Point monitors typically sample air from one point in space (e.g., via sampling tubes and pumps or via direct sampling). Alternatively, point monitors can sample multiple locations by employing a network of tubing. Point monitors can be positioned at a fixed location or can be mobile (e.g., positioned on a mobile sampling vehicle). Point monitors can be used to make physical, chemical, or spectroscopic measurements of the air being sampled.
Open-path monitors project an optical beam through space (e.g., along a monitoring path) so as to observe a continuum of points simultaneously. An open-path monitor can be bistatic or monostatic. A bistatic open-path monitor employs an optical source and a separate, down-range receiver. A monostatic open-path monitor employs a combined source/receiver and therefore also employs a down-range reflector (e.g., an array of mirrors). Because open-path monitors project a beam of electromagnetic radiation across space, open path monitors are typically limited to measurements made with optical spectroscopy.
When measuring relatively homogenous air masses (i.e., where the air mass along the optical beam is homogeneous), open-path monitors offer little advantage over point monitors. Indeed, open-path monitors are typically more complex and expensive than point monitors. That said, open-path monitors are superior when an air mass contains distinct plumes of pollutants (e.g., from a leak). An open-path monitor can detect a fugitive plume at any point along the optical beam. Open-path monitors can also provide near real-time monitoring. Therefore, open-path monitors are particularly suitable for fence-line monitoring (e.g., around the perimeter of an industrial facility).
An exemplary technique for operating an open-path monitor is ultraviolet differential optical absorption spectroscopy (UV DOAS). UV DOAS systems are described in U. Platt, Differential Optical Absorption Spectroscopy (DOAS), in 127 CHEMICAL ANALYSIS, AIR MONITORING BY SPECTROSCOPIC TECHNIQUES 27 (Markus W. Sigrist ed., 1994), which is hereby incorporated by reference in its entirety. UV DOAS systems are also described in E. D. Thoma & E. L. Thompson, Measurement of Low Level Air Toxics with Modified UV DOAS, Proceedings of the 99th Annual Conference of the Air & Waste Management Association (Jun. 20-23, 2006), which is hereby incorporated by reference in its entirety.
Other exemplary techniques include open-path Fourier transform infrared (OP-FTIR) spectroscopy, tunable diode laser absorption spectroscopy (TDLAS), and differential absorption laser (DIAL) spectroscopy. U.S. Patent Application Publication No. 2007/0045542, which is hereby incorporated by reference in its entirety, discloses an optical remote sensing monitor employing tunable diode laser absorption spectroscopy or open-path Fourier transform infrared spectroscopy.
These techniques detect pollution through speciation. In other words, these techniques detect specific compounds with spectroscopy. Speciation requires a high spectral resolution. Accordingly, these techniques require sophisticated hardware and software. High-performance equipment is generally expensive. Moreover, this high-performance and precise equipment lacks the robustness needed for long-term outdoor applications.
Accordingly, a need exists for a more robust open-path pollution monitor.